AMB says: "Differences between NwAvGuy's and my test results are due to several factors -- different equipment, different methodology, different assumptions, and a possibility that his sample of the Mini³ was not working properly."

This is a classic "hand waving" argument. It's like saying the Mustang does 0 to 60 MPH in 1 second flat and the reason Road & Track measured 6.5 seconds is they used a different track. That's not good enough. The correct numbers are the correct numbers. AMB either needs to show, with real engineering, real facts, and/or real math where his claims are valid, or he needs to stop publishing grossly misleading information. It's as simple as that.

If his RMAA test setup is unable to correctly measure his designs, he should trust the numbers of those who do have the proper equipment--not publish highly misleading numbers from his flawed test set up.

But all the testing in the world can't explain away his impossible crosstalk measurements. It's like saying pigs can fly because they're magically exempt from gravity.

I'd like to see AMB's dummy load and how it's wired. Something stinks here, and he seems to be making a pretty large excuse if something special needs to be done to get the "ideal" measurements. We're talking a -30dB different favoring the O2 on the dummy load I used. I got the same measurements as AMB when my dummy load was pieced together wrongly or without load.

If he isn't treating it like it's a resistive headphone attached then his measurements are pretty useless IMO.

Heck, if I understand this correctly he's also saying a regular interconnect cable will screw up the measurements . . . yet it's a portable amp? Anyone else see a gaping flaw in logic here?

I don't think the SR-71 has battery protection (someone please correct me if I'm wrong?) which is a big deal in my opinion. It could damage your headphones (and also the batteries).

It costs quite a bit more than the O2 and, based on the Stereophile measurements, the O2 will generally match or outperform it. JA only tested down to 150, and in one case 100 ohms. What about 25 ohm Denons? Or 60 ohm AKG K701s? Or 38 ohm HiFiMans? I have a feeling the O2 will easily better the SR-71 into those sorts of impedances.

It's (mostly)surface mount which has the advantage of making it smaller than the O2 but that also makes an amp much less DIY friendly.

all this hype but i still need to test it myself

also any words about vendors selling all the parts? that number 2 listed early? i just dont find it reasonable to pay double for the shipping than for the parts...

also any words about vendors selling all the parts? that number 2 listed early? i just dont find it reasonable to pay double for the shipping than for the parts...

Hopefully my "hype to measurements" ratio is better than most and there's lots of real science behind most of it. One of the main goals of the O2 was to avoid the more obvious flaws--like some of what you quoted above--with everything else I could find that was relatively affordable. See Going Shopping. But I won't deny I'm a "proud parent".

As for kits, etc, if you look at the Group Buy PCB list there are many suspiciously high quantities. At least one board buyer has already mentioned he will have extra builds "looking for homes". I suspect we'll see more of that once people start receiving the boards.

The O2 has been an all volunteer effort and its rather "organic". If you don't like the choices now for building one I would suggest waiting as I'm sure more opportunities will arise.

And thanks Shike. I agree if the load isn't properly applied that renders pretty much all the tests null, void, and highly misleading. It's black and white. Either the amp sees a 33 ohm load or it doesn't. Resistors are isolated floating devices. They're not referenced to anything. You can attach them wherever you want. Surely AMB understands that. So, I agree, something does stink.

More confusion is percolating elsewhere. One claim is my design goals for the O2 are somehow misguided and I sacrificed important aspects of the O2's design for a single end goal--namely impressive crosstalk. That's not the case but I can see how someone might get that impression given all the crosstalk discussion.

I have said all along I don't believe crosstalk needs to be especially impressive. Right in the O2 requirements it's specified at only 40 dB. The whole crosstalk issue has been front and center because I'm trying to show AMB's published specs and claims are wrong. Crosstalk just happens to be a way to do that without having to make any measurements. AMB's crosstalk numbers are literally impossible with the loads he claims to have used. And if the loads were wrong, that calls into question the rest of his measurements. The whole point is AMB is doing the DIY community a disservice by not publishing accurate measurements and specs. It would be no different if Stereophile tested one of his amps and found similar large discrepancies. AMB should get his own house in order before he starts throwing rocks at others.

The gain structure (i.e. putting the volume control "in the middle") of the O2 was mainly done for noise reasons--it has nothing to do with crosstalk. Many headphone amps have audible noise with many portable headphones--including the Mini3. By putting the volume control where I did, I solved a genuinely audible problem for many portable amp users.

The O2 doesn't use a 3 channel design or virtual ground for very real-world reasons like insufficient output voltage for many headphones, insufficient current for many headphones, shorter battery life, and high levels of inter-channel distortion. A proper star ground will always outperform a virtual ground in many ways--not just crosstalk.

There's also been a lot of claims that 16 ohm loads are not realistic and I'm crazy to even test at 15 ohms. Why then do so many portable amps and portable gear makers specify their power at 16 ohms? Even NuForce does. How am I supposed to verify their claims if I don't test at essentially the same impedance they do? There are plenty of high-end headphones under 32 ohms including 16 ohm Etymotics, 16-25 ohm Ultimate Ears, 25 ohm Denons, etc. And that doesn't include all the popular models from Skullcandy, Meelectronics, etc. which are almost entirely 16 ohm.

I've been accused of "dividing" the headphone community when all I'm trying to do is help educate more people about the objective side of headphone audio and verify objective claims made by others. Long before I came along there has been the "great debate" around subjective vs objective viewpoints. I personally believe the objective side of that discussion still hardly even has a foot in the door. Accurate objective information can help people make more informed buying decisions and save a lot of people a lot of money. What's so awful about that? How is it different than what Tyll is doing at InnerFidelity or John Atkinson does for Stereophile?

I've been accused of being "confrontational" but it's hard not to be when you test a product, find obvious problems, and you're immediately attacked as I was by NuForce fans, Schiit fans, and AMB fans. I can't change the numbers when I test things. If the Ford is faster than the Chevy, I'm sorry for the Chevy fans but please don't shoot the messenger. All I did was test both of them in the exact same way. If the Chevy fans start throwing rocks I'm going to defend my test results.

I've been accused of excessive "negativity" but I've given lots of products very favorable reviews. I also have written a lot of educational articles that have been very well received (and about to publish more). And the O2 is designed to be a very positive thing. Instead of just complaining about problems with headphone amps I decided to offer a solution. For free. What's negative about that? I'm also looking for a DAC I can hold up as being a great value for the money.

Unfortunately not everyone is going to play nice together in the audiophile world. There are too many strong conflicting opinions and there's too much money behind a lot of those opinions. I'm just trying to focus on the objective side of things.

I'd like to see AMB's dummy load and how it's wired. Something stinks here, and he seems to be making a pretty large excuse if something special needs to be done to get the "ideal" measurements. We're talking a -30dB different favoring the O2 on the dummy load I used. I got the same measurements as AMB when my dummy load was pieced together wrongly or without load.

If he isn't treating it like it's a resistive headphone attached then his measurements are pretty useless IMO.

Heck, if I understand this correctly he's also saying a regular interconnect cable will screw up the measurements . . . yet it's a portable amp? Anyone else see a gaping flaw in logic here?

Yeah, simplest explanation is just that whatever load the amp was seeing was a lot higher than 33 ohms. Seems like a quick sanity check would be just to measure the voltage across the resistor when on the RMAA calibration screen, which even a cheap multimeter would be able to confirm. If it's not an order of magnitude within about 1V then you're probably in trouble.

I think he's saying that a regular interconnect cable from the amp output into the sound card line in causes problems (no problems loaded), but maybe I should refrain from continuing this line of speculation.

NwAvGuy, I forget about the timeline of changes on the O2. At one point there was an issue with noise with very high output impedance sources (irrelevant for me), and there was a 350 mV peak turn-on transient (likewise not much of anything). There were also a few minor parts changes in the past couple of weeks.

Were those two issues being addressed? If so, what's the difference now, just out of curiosity? Thanks.

NwAvGuy, I forget about the timeline of changes on the O2. At one point there was an issue with noise with very high output impedance sources (irrelevant for me), and there was a 350 mV peak turn-on transient (likewise not much of anything). There were also a few minor parts changes in the past couple of weeks.

Were those two issues being addressed? If so, what's the difference now, just out of curiosity? Thanks.

The most current design documents are all dated August 25th and have all the changes.

The high impedance source issue was resolved by suggesting people ground the enclosure to the input ground (details here) and by removing the 0.22 uF cap at the AC power input.

The transient issue was optimized by lowering C16 and C21 to 0.022 uF and increasing C22 (now C1) to 1.0 uF.

I also tested the expensive Vishay low noise mil-spec resistors against the less expensive Vishays and found no meaningful difference. So I moved those to the comments section of the BOM as being an option and made the cheaper resistors the primary parts.

I changed the Fairchild negative regulator to an On-Semi part as the Fairchild was discontinued and its replacement far more expensive. The On-Semi part also has better specs.

The other parts changes were mostly due to changing stock and prices at Mouser.

By the way there exist IEMs with nominal impedance under 16 ohms. Some listed at 16 ohms are undoubtedly below that at some frequencies of course. This is just through a quick check through the one mega-review thread at HF (the one with 191 mini-reviews/tech specs and counting), so this list is probably not exhaustive.

@mikeaj, agreed. My 21 ohm Super Fi Pro 5's dip to 10 ohms. I'm sure there are plenty of examples and likely even some that might need 400 mV or more. Any portable device with a headphone jack should be designed for 16 ohm headphones. I'd venture to guess if you add up the total sales, more 16 ohm headphones are sold than any other impedance (even if most of them are under $50). A quick search on Amazon shows a lot of the top sellers with huge numbers of reviews are 16 ohms.

I've finished updating the O2 Measurements where applicable with the V1.1 latest board, at the new gains. About a third of them changed enough to warrant updating and the other two thirds were so close (within normal measurement tolerances) I didn't bother. I also found (as usual) and fixed several typos, formatting issues, etc. along the way.

I've switched to the industry standard dBv for noise measurements. Someone emailed me and accused me of faking the noise measurements by using a different reference. By switching the dScope to dBv there is no user adjustable reference so the numbers can't be doctored. And, given all my critics, I'd have to be really foolish to fake any of the measurements as I'm sure the O2 will be measured by others.

The unit dBv is always referenced to 1 volt RMS. The reason I used to use dBr referenced to 400 mV is some portable devices can't manage 1 volt. But it's easy to convert between the two. The details are in the article.

Bottom Line: Most things are slightly better at the lower gain and I think only one measurement, the wideband THD at 1 Khz at 15 ohms, was barely worse.

Seems buying the O2 pre-built is cheaper than trying to source parts myself and putting one together, which kind of sucks really since I was itching to bust out the soldering iron. Shipping costs more than the sum total of the parts from Mouser alone (ie No enclosure, panel, wall-wart)....

Seems buying the O2 pre-built is cheaper than trying to source parts myself and putting one together, which kind of sucks really since I was itching to bust out the soldering iron. Shipping costs more than the sum total of the parts from Mouser alone (ie No enclosure, panel, wall-wart)....

IMPORTANT UPDATE: Apparently even an enormous distributor like Mouser can't keep up with O2 demand. They've sold out of the main volume pot and all the acceptable substitutes. And more are 9 - 11+ weeks away. So I'm working on revising the PC board to accommodate a different series of pots as well as the original part but it's proving to be tricky.

A lot of DIY projects require purchasing from 3 -5 sources. A major goal for the O2 was to have everything on the board available from Mouser to save on shipping costs--at least in North America. For those outside North America, the group buys (so far one for Europe and one for Australia/New Zealand) should help a lot.